This invention relates in general to optically addressed light valve systems and more specifically to real-time reflective light valve systems and processes for using the light valves.
With the advances in imaging process techniques, various types of optically addressed light valves are now practical. Desirably, such light valves should permit read-out during the write-in cycle. The intensity of the read-out light should exceed the write-in intensity, which preferably is small, by many orders of magnitude.
Many types of light valves are described in the patent and scientific literature. In general, such light valves comprise a photoconductive layer in series with an electro-optic layer such as a liquid crystal layer, ferroelectric layer or the like. The photoconductive layer is placed under an electrical bias and light imagewise incident on it effectively causes its electrical resistance to decrease in the exposed areas. This results in imagewise changes in the electric field across the electro-optic layer which in turn can be read out or projected. Optically addressed light valves based on photoconductor controlled liquid crystal layers offer many attractive features such as high light sensitivity, excellent resolution, small power consumption and low cost.
It has heretofore been accepted that prevention of image washout by the read-out light would require either read-out with light of wavelengths to which the photoconductor is insensitive or, alternately, insertion of an optical blocking layer between the electro-optic medium and photoconductor. Both alternatives are discussed in the literature. In an article entitled "AC Liquid-Crystal Light Valve" by T. D. Beard, W. P. Bleha and S. Y. Wong which appeared in Applied Physics Letters of Feb. 1, 1973, cadmium sulfide is used as the photoconductor, and a light crystal as the electro-optic medium. To separate the write-in light from the read-out light, an optical blocking layer of cadmium telluride is used in conjunction with a dielectric mirror. Similarly, in U.S. Pat. No. 3,824,002, a cadmium telluride layer is used to block light that could discharge a cadmium sulfide photoconductor. Although cadmium telluride blocks out visible light, including red light, it may form a heterojunction with the underlying photoconductor which may cause problems of an electrical nature. Furthermore, cadmium telluride is difficult to deposit.
Although excellent imaging may be obtained with optically addressed light valves, read-out of photoconductor layers such as AS.sub.2 Se.sub.3 require a cut-off filter for the read-out light which allows transmission of only wavelengths below about 6,000 angstroms (e.g. see U.S. Pat. No. 4,037,932 and "Ultralow-voltage image intensifiers", by Haas et al, Applied Physics Letters of Nov. 15, 1976). It has been found that for color images, the spectrum of the read-out light that strikes the mirror must be of a broad spectrum in order to produce an amplified color image having true red hues. As is known in the art, color images are formed by using two or more light valves and combining colors, e.g. see U.S. Pat. No. 4,127,322, the entire disclosure of this patent being incorporated herein by reference. Thus, for true color read-out images it is essential that the red region of the spectrum not be filtered out from the read-out light before it reaches the light valve. If the red region of the spectrum is filtered from the read-out light before it reaches the light valve, the final reflected image takes on an undesirable orange-red appearance in the regions where it should appear red. If the red region of the spectrum of the read-out light passes through the photoconductor layer and the photoconductor layer is sensitive to red (e.g. photoreceptor contains arsenic and selenium), the final reflected image takes on a washed out appearance. Therefore, when photoconductor layers sensitive to red are employed in light valves, red light (e.g. light having a wavelength between about 6,000 angstroms and about 7,000 angstroms) erases the image on the photoconductor layer unless a filter is used on the read-out light or an effective mirror and/or blocking layer are employed.
In new and growing areas of this technology, new methods, apparatus, compositions and articles of manufacture are often discovered for the application of the new technology in new modes.